Pool Leak Source

I have a plaster pool that is dropping 2-3" each 24 hours. The same rate continued after the water level dropped below the skimmer and the two outlets. There are a few cracks which well could have been the problem but I dye tested(using food coloring) the area around all cracks and there is no dye getting sucked into the cracks.

It would seem the only remaining potential sources are the light or main drain. The water level is now about half way down the light fixture. I have dye tested the light and did not see any suction of dye. I have read that a dye test may not reveal a fixture leak unless the fixture is pulled out(I did not pull out the fixture). Now, I just noticed that there is water in the light fixture(i.e. behind the glass cover) that is at a level about 3" higher than the pool. Any thoughts? Thanks.